OM!
Honestly the only relevant characters in terms of Pokemon's overall story is Ash, Misty (minor), Brock (minor), May, Dawn, Cilan (minor), and Clemont (minor). And depending how the story turns out maybe Lillie, but once again she'd be minor. Why is that? Because honestly the story of a Pokemon is a one trainer's journey. You only need one main character to be the viewer's viewpoint into the world's main thing, Pokemon battling. Everything thing else like different battling styles, other mechanics, world lore, etc. can be handled by one off characters. Reason I say Misty, Brock, Cilan, and Clemont have minor relevance is because they are Gym Leaders, they are bosses in the games you needed to defeat to progress and that was brought over to the anime (Iris and the majority of the Alolan crew being the exception, Iris isn't a Gym Leader yet and they completely removed the concept of Trail Captains from the anime).
May and Dawn had a unique position, Gen III and IV had Pokemon Contests which, at the time, were treated as as alternate activity to Pokemon battling. This meant in the anime they could have an additional main protagonist with relevance. They tried doing this with Misty and Brock, Misty representing the "collector" and Brock the "breeder"... but there wasn't really anything to enforce those (especially breeding, at least on the level of Brock's goal; also since the main point of the games was catching them all it made Misty's goal seem small in scale as she only wanted to catch Water-type Pokemon). But after Gen IV the characters were given nothing to work with or nothing that was in the game. Serena was given Pokemon Showcases which were Pokemon Contests-lite as the main point of them was to show off Serena as they wanted her to be a waifu. We'll see where Lillie's development goes but without being Trial Captains the others in the Alolan Crew are now also irrelevant.
That said, a one trainer show would be boring pretty fast, you need additional characters with different personalities, goals, and skills to keep things interesting. The majority of Ash's companions serve more to be an extension of Ash's capabilities, what he can't do they can do and learn from them (well, in theory, being they hit the reset button almost every generation now). Also, if the writing crew gave a mulch, they could have made more interesting characters that went with a mechanics or even plot of that generation. Well, actually, they did try between Gen I and IV, but in Gen V they gave up producing a subpar region arc, and Gen VI onward decided to make the anime they want not really caring if it was Pokemon or not.
She wasn't the greatest, but I don't think that's why she didn't return. It's because the writing team stopped writing a Pokemon series a while ago. They wanted to focus on Shonen battling (complete with super transformations) and idol making. Follow a show tradition by bringing back the previous female companion for a few episode cameo? Well Iris wasn't that popular and now we have waifu Serena, why do we need anyone else? Actually, scratch that, Yokai Watch! is doing pretty good so we're changing Pokemon into a slice of life. Cut out Serena, ship her off, she doesn't fit. We'll just give Ash a harem of the new girls.
Course, why is Misty & Brock being brought back then? Well, nostalgia obviously (and that comes with a ratings boost). That's why Charizard was brought back. I also wonder if it's because of their exclusion from the movie. "Probably not, their episode were probably planned during the production of the movie". True, though maybe they had a moment of thinking and realized the fans wouldn't be happy of them chopping out the characters who scored first and second place on the "Ash's Companion" popularity poll so planned these episodes just in case of backlash. Which they got. Though now that makes me wonder if there's alternate episodes/scripts which has Ash visiting the new movie characters in case they did go over well.